Internal corrosion takes place due to poor quality of feed water or inadequate boiler water treatment. Boiler water may contain dissolved air or gases such as oxygen and carbon dioxide.
Oxidation and Pitting Corrosion
Oxygen in water causes oxidation of steel, resulting in oxidation corrosion.
4Fe + 3O2 ——> 2Fe2O3
This is general corrosion, which causes pitting. In order to prevent this:
- Use mechanical deaerators to remove dissolved gases from feed water
- Use vacuum condenser and keep hot well closed
- Use closed circuit system
- Dose chemicals such as HYDRAZINE (It reacts with oxygen and releases nitrogen and water)
N2H4 +O2 ———-> N2 + 2H2O
Carbon dioxide dissolved in water forms carbonic acid.
CO2 + H2O ——–> H2CO3
Chlorides in feed water forms hydrochloric acid.
MgCl2 +2H2O ——–> Mg(OH)2 + 2HCl
Fe + 2HCl ———> FeCl2 + H2
In order to counteract the acidic corrosion, alkaline compounds such as sodium hydroxides are added to boiler water.
MgCl2 + 2NaOH ———> Mg(OH)2 + 2NaCl
Corrosion Due to Scale Formation
Scale formations results in heavy pitting corrosion. Phosphate treatment in boiler water reduces this type of corrosion.
CaSO4, MgSO4, CaCO3 are scale forming compounds
In order to control the corrosion, tri sodium phosphate, Na3(PO4)2, is dosed.
3CaSO4 + Na3(PO4)2 ———> Ca3(PO4)2 + 3Na2SO4
By adding boiler coagulant chemicals, these products from non-sticky sludge and are washed away while regular bottom blow down.